Firstly, you will need to find an available domain name you would like to use. For this example, I will use "myownns.net". (Hey...and its available

Firstly, you need to have nameservers for the domain you want to use. So, in whm, create an account using this domain name.

[edit]
wow...3 firstly's.... make that "firstly", "secondly" and "thirdly"

When registering a domain name, you normally need to specify nameservers for the domain. If you do this, your private label nameserver will then also contain another domain name, (e.g. nameservers for myownns.net will be ns1.yoursp.com and ns2.yoursp.com). but most registrars give you an option to have no references to other domains. I use godaddy.com for registrations, and when registering a domain. When prompted for nameservers, look for an option that lets you define your own hosts on that domain. That way, you register the domain, using ns1.myownns.net and ns2.myownns.net, and you have the option to specify the IP's for this (the IP's will be that of your server). Now you have a domain registered that does not refer to any other domain name...

You also have an option of making your domain private at this point, which costs a few $'s a year. This will hide ALL your registrant information, through the use of a third party service, ensuring total privacy for all your resellers. Check with your registrar on how to do this.

Now whm configuration only starts... :p

There are 2 places you need to define your nameservers:
Firstly, go to Basic cpanel/WHM setup (under Server Configuration). There you will find nameserver entries - add ns1.myownns.net and ns2.myownns.net. Be sure to create DNS entries for this, using DNS manager -> Edit DNS zone (or wait on this, there is another way..).

Also give your server a name on the domain - e.g. alpha.myownns.net.

Next, you can configure "Nameserver IP's" in whm. This will configure an IP for use as a nameserver. From what i can remember, this uses the first available IP address as you enter host names - . Assign for your nameserver's on the domain - ns1.myownns.net first and then ns2. If you need to manually edit this, you will need to ssh in to your box as root, and edit /etc/nameserverips.

Finally, go to your "Edit dns zone" in whm, and make sure you have dns entries for all the hosts you have specified - ns1, ns2 and alpha / whatever you named your server in the Basic cpanel/WHM Settings section.

Lastly, click the "Hostname" option under networking setup. set it to your new hostname.

One more thing you would want to do is to get reverse DNS set up - this is a requirement by certain mail servers that your users will be sending to, and by default reverse DNS is probably pointing to your providers domain. You should send a support request to have reverse DNS (a.k.a PTR records) for your IP's set to your nameserver records (or other forward DNS records that resolve to your server).

Voila - you have now a complete private label server. Even telnet yourserver 25 will use your private label domain name in smtp greeting, as well as POP and it is almost impossible to determine "who you are" from the domain name.

Hope this helps...

One more quick note - check your resolver configuration, and make sure you set this to YOUR SERVICE PROVIDER'S DNS SERVER - if you break your nameserver, then your entire cpanel server becomes unusable, as it depends on itself to resolve if your resolver configuration points to itself.